The key components of a base terminal for free-space optical point-to-multipoint communications are transmitters emitting in several given directions separate optical beams modulated, in a general case, by different transmit signals, and receivers, based on photo-detectors and used for separate reception of optical signals coming from different directions. In case of multiple subscribers to be serviced from one hub, point-to-multipoint free-space optical communication systems provide considerable advantage over use of a “multi-channel principle”, i.e. deployment of multiple similar-type channels operating point-to-point.
The use of a “multi-channel design” in a free-space optical communication system increases its mass, size, and cost approximately N-fold where N is the number of directions (of separate optical beams) needed to provide communications for N subscribers with one hub, because everyone of the similar-type channels comprises all components and subsystems needed for operation, including the ones which in point-to-multipoint systems are represented by a shared single unit supporting operation of all channels.
A free-space optical communication system is known (see the description of the Great Britain Patent No. 2,180,116). The system is implemented as a plurality of photo-detectors and a plurality of radiation sources placed on a curved surface, e.g. on a hemisphere. The system has the following drawbacks: complicated alignment of sources relative to every corresponding receiver; limited application because of distance limitation caused by large beam divergences which, in turn, causes large beam spread losses. To increase the distance of communication it is necessary to reduce a receive signal bandwidth, because the lower photo-detector input signal supports the narrower bandwidth allowed for reliable operation of the channel.
A free-space optical communication system is known from the description of the U.S. Pat. No. 5,909,296. The system comprises light sources with light beam modulation means, and one or more optical receivers with demodulation means. To reduce the beams divergence, each of the sources is provided with a micro-lens. The sources are immovable relative to the lenses, thus precluding alignment of the beam width and/or direction in case of a change in distance and/or angle between the receive and transmit terminals. Therefore, despite of a beam-width reduction due to use of the lenses, the system efficiency is low.
From the description of the U.S. Pat. No. 3,713,163, a system is known for communication with a plurality of moving objects. The system comprises radiation sources and receivers placed in the paraxial area at the focal surface of a single focussing device, e.g. a parabolic reflector or a lens. Several sources and receivers are implemented so that they can be rotated around the focussing device axis, providing their coupling with a selected object. A drawback of this system is the use of a shared focussing device with narrow field of view, thus limiting the solid angle wherein the plurality of objects can be located, by the paraxial area.
A more recent technical development in a free-space optical communication system is described in 00/48338. The system proposed in WO 00/48338, if implemented for two-way communications, comprises a plurality of radiation sources and a plurality of photo-detectors placed in the focal surface of a shared wide field of view lens system of the transmit/receive base terminal, as well as sources and photo-detectors installed on several remote subscriber terminals. In accordance with the description in WO 00/48338, at the transmit/receive base terminal the transmit and receive optical beams are passing through the same shared lens system. This lens system is used as a common optical antenna providing concentration on the base terminal photo-detectors of light beams intended for those detectors. The common lens system also serves as a collimating device reducing the divergence of light emitted by the sources and output as light beams targeted at the distant subscriber terminals. This communication system has the following drawbacks.
Firstly, the use of a common multi-component lens system in the transmit and receive base terminal optical paths requires spatial superposition (or, at least, optical matching) of the source and photo-detector providing two-way communication to and from the corresponding subscriber terminal. Both the source and photo-detector must be spatially, or optically, combined in the same area of a lens system focal surface corresponding to a particular subscriber terminal related to the source and the photo-detector.
To provide such spatial superposition of the source and photo-detector within the lens system focal area allocated for a particular subscriber, these elements must be of a size several times less than the lens system's point spread function's transverse size, which in general is difficult to achieve and may be prohibitively expensive. The description suggests use of optical superposition, where the specular reflections of the source and of the photo-detector are spatially superimposed by means of a beamsplitter, however, that approach requires additional expenses. At high density of components (sources and photo-detectors) in the focal surface of the shared lens system the beamsplitters would need to be so compact that it may be technically nonrealistic to implement such a design.
Another disadvantage is that the shared lens system used both as an optical antenna (in a receiving mode of operation) and as a collimating device (in a transmitting mode of operation) significantly reduces flexibility in employing efficient technologies for individual shaping and attenuation/amplification of the base terminal output optical beams to adjust the beams' divergence, average size of their transverse intensity distribution non uniformity, and output power to a particular operating distance, optical power losses in a free-space optical path and a subscriber terminal aperture size. The shared lens system also complicates the individual targeting of the base terminal output optical beams, including directing several output beams carrying the same information stream at one receive aperture of a subscriber terminal intended for that information.
Furthermore, to achieve wide field of view of the shared lens system proposed in the known communication system it is implemented consisting of at least two lenses,—a wide-angle lens and a second “focussing” lens (the term used by the authors of WO 00/48338). A common drawback of such multi-component lens systems is that the input optical aperture (pupil) is considerably smaller than the first lens diameter. As a result, to achieve a pupil diameter of 50–150 mm needed for many practical applications, it may be necessary to use a first lens having a diameter significantly larger than this value. Lens systems having diameters comparable or larger than 200–300 mm are expensive, and their use may require impractical weight and size in the base terminal.
The above conclusion regarding the weight and size of the base terminal is based on the following considerations. The wide-angle objective lenses may be typically characterized by the relation D′/F=1, where D′ is the objective lens diameter and F is its focal distance. Second, it is known that for multiple-lens systems
            d      ′        F    =      C                  tg        ⁡                  (                      γ            /            2                    )                    ·                                    F            /            100                    ⁢                                          ⁢          mm                    
where d is the pupil diameter, γ is the angular field of view, and the C parameter for different systems lies within 0.22÷0.24 (see . C..  . M., “”, 1978, c. 296). It follows from the above:
            D      ′              d      ′        >                    tg        ⁡                  (                      γ            /            2                    )                    ·                                    F            /            100                    ⁢                                          ⁢          mm                      C  
At γ=120° and F=150 mm the formula gives D′/d′>10.
Thus, in the wide-angle multi-component lens system the diameter of at least one lens may be an order of magnitude larger than the pupil diameter, causing corresponding increases in the terminal size and weight. Such an increase may be impractical if an attempt is made to achieve d′ values of 50/150 mm necessary for some practical applications in free-space optical communication.